Noun | 1. | fetoprotein - any of several antigens that occur naturally in the fetus and sometimes in adults with cancer AFP, alpha fetoprotein, alpha foetoprotein - an abnormally large amount of this fetoprotein in the fetus can signal an abnormality of the neural tube (as spina bifida or anencephaly) antigen - any substance (as a toxin or enzyme) that stimulates an immune response in the body (especially the production of antibodies) |